1. The Field of the Invention
This invention relates to geotechnical sealing materials and, more particularly, to novel systems and methods for reliable testing of shear testing properties of geotechnical materials and geotechnical synthetics.
2. The Background Art
The term "geosynthetics" is used to describe a broad group of flexible construction materials that include geotextiles, geogrids, geonets, geomembranes, and geosynthetic clay liners. These materials are used in various civil engineering applications for separation, filtration, reinforcement, transmission, stress adsorption, and barrier functions.
Geosynthetics that are commonly used for waste or fluid containment include geotextiles, geomembranes, geonets, and GCLs. Many waste or fluid containment structures are constructed of layers of geosynthetic materials or geosynthetics against natural soils or aggregates. As a result, the frictional properties between materials can become extremely important, especially if the structures are constructed on slopes.
The use of geosynthetics in geotechnical and environmental construction projects has dramatically increased in recent years as the advantages of the geosynthetic products have been demonstrated. The use of a manufactured sheet-type geosynthetic product is often in a layered system where the interface properties can become very important. The use of multi-layered geosynthetic liner systems for landfills is an important example. Construction of landfill liners and covers with moderate to steep slopes has raised concerns about stability against sliding. Due to their low friction angles, the interfacial shear strength properties of different geosynthetic interfaces are integral parameters for design.
Geotextiles are very versatile materials that can be used for separation, filtration, reinforcement, transmission usually in combination with a geonet, and as a barrier usually impregnated with asphalt or a similar waterproofing material. Geotextiles are classified by both their base polymer and structure. The most common base polymers for geotextiles are polypropylene and polyester. The two primary geotextile structures are woven and non-woven. The main distinction among various woven geotextiles is the type of yarn used. The most common yarn types are slit tape, monofilament, and fibrillated.
Geomembranes are very low permeability synthetic membrane liners or barriers used in civil engineering projects for fluid or waste containment. They are typically used for liquid or vapor barriers. They are used extensively in environmental engineering applications such as waste containment as part of a mandated liner system.
There are three categories of polymers that are used for geomembranes: thermoset elastomers, thermoplastics, and bituminous materials. The manufactured liners of primary interest for interface testing are the thermoplastic geomembranes since they are the most extensively utilized liners. The thermoplastic membranes include high density polyethylene (HDPE), low density polyethylene (LDPE), chlorinated polyethylene (CPE), chlorosulfonated polyethylene (CSPE), ethylene interpolymer alloy (EIA), and polyvinyl chloride (PVC). The HDPE liner material is the most frequently used liner.
Geosynthetic clay liners are factory fabricated rolls of bentonite placed between two geotextiles or bonded to a geomembrane using an adhesive. The GCL is considered to be a geocomposite since it is fabricated from a geosynthetic material and bentonite clay. The bentonite clay is the critical component of a GCL since it is the source of the very low hydraulic conductivity of the GCL. When exposed to water, the bentonite will adsorb the water and swell. The resulting hydrated GCL has a hydraulic conductivity in the vicinity of 1.times.10.sup.-9 cm/sec. The carrier material for a GCL can be a woven or non-woven geotextile or a geomembrane, depending on the design and construction of the GCL.
Testing of GCLs in the hydrated condition requires a period of time (often 1 to 3 days) where the dry bentonite clay in the GCL is allowed to hydrate in the presence of water or other fluid, under a specified normal load, before testing. In the process of hydration, the bentonite adsorbs water and can expand to several times it's initial volume due to the adsorbed water. Testing of the hydrated GCLs requires first hydrating the sample, then testing. Some conventional direct shear machines are configured for hydration at a known normal load away from the test device, transferring the sample to the test device, and then performing the test. The preferred method is to hydrate the sample under a known load in the test device and perform the test without disturbing the sample. Some direct shear machines are configured to hydrate and test in the test device. The Cylinder Direct Shear can be used with either method.
The interest in peak and residual friction angles increased significantly due to the 1988 Kettleman Hills waste landfill slope failure, where a slope stability failure occurred in a hazardous waste landfill (27 m or 90 ft high) in which lateral displacements up to 10.6 m (35 feet), and vertical settlements of up to 4.2 m (14 feet) were measured (Mitchell, et al., 1990). Failure developed by sliding within the composite, multi-layered geosynthetic-compacted clay liner system under the hazardous waste fill. Several geosynthetic-geosynthetic and geomembrane-compacted clay liner interfaces were found to have low frictional resistance.
Investigations used small 2.8 inch shear boxes with a relative shear displacement of less than 0.3 inches and pullout box tests with larger relative shear displacements in excess of 3 inches. The testing revealed that a peak friction was typically encountered at a small relative displacement, followed by a decrease in friction with increasing displacement. Peak strengths occur at very small relative interface shear displacements. If such displacements are exceeded by deformations occurring during construction and placement, the residual interface shear strength behavior is actually unknown.
The residual strength of a geosynthetic interface is an important design parameter but various test methods have differing relative displacements. Residual shear strengths (or residual friction angles) have been reported for displacement of less than one cm for small direct shear test displacements of 7 to 10 cm or more for large direct shear and pullout tests, and greater than one m for some torsion ring tests. Reported friction properties vary widely with different materials, conditions, and test methods. What is needed is a repeatable, reliable method and apparatus for determining an interface friction angle for each interface representative of field conditions and materials. Each current method has advantages and disadvantages. The primary disadvantage with the direct shear test is the limited displacement available.
A 0.3 m direct shear device for testing geosynthetics has since been standardized since 1992 through the development of American Society for Testing and Materials (ASTM) Standard D-5321, Standard Test Method for Determining the Coefficient of Soil and Geosynthetic or Geosynthetic and Geosynthetic Friction by the Direct Shear Method.
The reported shear strength parameters are typically the peak shear strength and the shear strength at the end of the test, which is often called the residual shear strength. For many geosynthetic materials, the residual friction angle and residual shear strength varies with displacement, with the shear strength decreasing with increased displacement. As a result, shear strengths are often reported at a fixed displacement or percent deformation, without determining the final or true residual friction.
It should be emphasized that most of the available shear testing methods have a relatively small displacement range (typically from 0.3 inch up to 3 or 4 inches, while real world failures often measure displacements in ranges of feet and sometimes tens of feet. A test method that could duplicate this scale of displacement would provide more reliable and more believable shear strength test results.